Numerous members of the cytochrome P450 (CYP) superfamily are induced after exposure to a variety of xenobiotics in human liver. We have gained considerable mechanistic insights into these processes in hepatocytes and multiple ligand-activated transcription factors have been identified over the past two decades. Families CYP1, CYP2 and CYP3 involved in xenobiotic metabolism are also expressed in a range of extrahepatic tissues (e.g. intestine, brain, kidney, placenta, lung, adrenal gland, pancreas, skin, mammary gland, uterus, ovary, testes and prostate). Since the expression of the majority of the isoforms appears to be very low in the extrahepatic tissues in comparison with predominant expression in adult liver, the role of the enzymes in overall biotransformation and total body clearance is minor. However, basal expression and up-regulation of extrahepatic CYP enzymes can significantly affect local disposition of xenobiotics or endogenous compounds in peripheral tissues and thus modify their pharmacological/toxicological effects or affect absorption of xenobiotics into systemic circulation. The goal of this review is to critically examine our current understanding of molecular mechanisms involved in induction of xenobiotic metabolizing CYP genes of human families CYP1, CYP2 and CYP3 by exogenous chemicals in extrahepatic tissues. We concentrate on organs such as the intestine, kidney, lung, placenta and skin, which are involved in drug distribution and clearance or are in direct contact with environmental xenobiotics. We also discuss single nucleotide polymorphisms (SNPs) of key CYPs, which at the level of transcription affect expression of the genes in the extrahepatic tissues or are associated with some pathophysiological stages or disorders in the organs.

The breast cancer resistance protein (BCRP/ABCG2) is an ATP-binding cassette drug efflux transporter that extrudes xenotoxins from cells, mediating drug resistance and affecting the pharmacological behavior of many compounds. To study the interaction of human wild-type BCRP with steroid drugs, hormones, and the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP), we expressed human BCRP in the murine MEF3.8 fibroblast cell line, which lacks Mdr1a/1b P-glycoprotein and Mrp1, and in the polarized epithelial MDCKII cell line. We show that PhIP was efficiently transported by human BCRP in MDCKII-BCRP cells, as was found previously for murine Bcrp1. Furthermore, we show that six out of nine glucocorticoid drugs, corticosterone, and digoxin increased the accumulation of mitoxantrone in the MEF3.8-BCRP cell line, indicating inhibition of BCRP. In contrast, aldosterone and ursodeoxycholic acid had no significant effect on BCRP. The four most efficiently reversing glucocorticoid drugs (beclomethasone, 6α-methylprednisolone, dexamethasone, and triamcinolone) and 17β-estradiol showed a significantly reduced BCRP-mediated transepithelial transport of PhIP by MDCKII-BCRP cells, with the highest reduction of PhIP transport ratio for beclomethasone (from 25.0 ± 1.1 to 2.7 ± 0.0). None of the tested endogenous steroids or synthetic glucocorticoids or digoxin, however, were transported substrates of BCRP. We also identified the H₂-receptor antagonist drug cimetidine as a novel efficiently transported substrate for human BCRP and mouse Bcrp1. The generated BCRP-expressing cell lines thus provide valuable tools to study pharmacological and toxicological interactions mediated by BCRP and to identify new BCRP substrates.

Aryl hydrocarbon receptor (AhR) is an important transcriptional regulator of drug metabolizing enzymes that dominantly controls the expression of cytochrome P450 CYP1 family genes and some phase II enzymes. AhR also has many endogenous functions including cell cycle control, immune response, and cell differentiation. In addition, AhR is well-known to be involved in chemicallyinduced carcinogenesis. AhR is activated by a variety of endogenous and exogenous ligands. While exogenous activation of AhR has deleterious effects on human organism, sustained activation of AhR by endogenous ligands is indispensable for proper cell functions. Therefore, the effects of exogenous and endogenous ligands on AhR resemble the Dr. Jekyll and Mr. Hyde story. The aim of the current paper is to summarize and update the knowledge on exogenous and endogenous AhR ligands.

In our study, we tested the hypothesis whether valproic acid (VPA) in therapeutic concentrations has potential to affect expression of CYP3A4 and MDR1 via constitutive androstane receptor (CAR) and pregnane X receptor (PXR) pathways. Interaction of VPA with CAR and PXR nuclear receptors was studied using luciferase reporter assays, real-time reverse transcriptase polymerase chain reaction (RT-PCR), electrophoretic mobility shift assay (EMSA), and analysis of CYP3A4 catalytic activity. Using transient transfection reporter assays in HepG2 cells, VPA was recognized to activate CYP3A4 promoter via CAR and PXR pathways. By contrast, a significant effect of VPA on MDR1 promoter activation was observed only in CAR-cotransfected HepG2 cells. These data well correlated with up-regulation of CYP3A4 and MDR1 mRNAs analyzed by real-time RT-PCR in cells transfected with expression vectors encoding CAR or PXR and treated with VPA. In addition, VPA significantly up-regulated CYP3A4 mRNA in primary hepatocytes and augmented the effect of rifampicin. EMSA experiments showed VPA-mediated augmentation of CAR/retinoid X receptor α heterodimer binding to direct repeat 3 (DR3) and DR4 responsive elements of CYP3A4 and MDR1 genes, respectively. Finally, analysis of specific CYP3A4 catalytic activity revealed its significant increase in VPA-treated LS174T cells transfected with PXR. In conclusion, we provide novel insight into the mechanism by which VPA affects gene expression of CYP3A4 and MDR1 genes. Our results demonstrate that VPA has potential to up-regulate CYP3A4 and MDR1 through direct activation of CAR and/or PXR pathways. Furthermore, we suggest that VPA synergistically augments the effect of rifampicin in transactivation of CYP3A4 in primary human hepatocytes.